1. Field of the Invention
The disclosure relates to a ZnO-based thin film transistor (TFT) and a method of manufacturing the same.
2. Description of the Related Art
Televisions (“TV”s) are the main applications for the rapidly progressing field of flat panel displays. While liquid crystal displays (“LCD”s) are the most widely used displays for TVs, many attempts have been made to use organic light-emitting diodes (“OELD”s) for TVs. The development of displays for TVs has moved toward a focus on the incorporation of large screens, digital information displays (“DID”), low cost, as well as a high level of quality in the moving image, resolution, brightness, contrast, and color production, each of which are important considerations in the market. To this end, the substrate should be increased in size, and a high quality thin film transistor (“TFT”) should be used as a display switching and driving element without increasing the overall cost. Given this trend, an effort is needed to develop TFTs for high quality displays that can be manufactured at a low cost.
Amorphous-silicon (“a-Si”) TFTs, which can be uniformly formed at a low cost on a large substrate up to 2 meters (m) wide or larger, are now widely used as driving and switching elements for displays. However, as the displays become larger the driving and switching devices are also desired to have higher performance in order to ensure a high quality image, and as such, the application of existing a-Si TFTs is limited due to their having a low electron mobility of approximately 0.5 centimeters squared per Volt second (cm2/Vs). Accordingly, high performance TFTs having an electron mobility that is higher than the electron mobility of the a-Si TFTs, need to be developed. In addition, the a-Si TFTs have another problem in that the performance of the a-Si TFTs degrades as they are operated over a long period of time, thereby lowering their reliability. For this reason, the application of a-Si TFTs to OLEDs, particularly as compared to LCDs, is difficult since OLEDs emit light using a continuously applied current.
Since poly-silicon (“poly-Si”) TFTs have a much higher performance than that of a-Si TFTs (i.e. a high mobility of tens to hundreds of cm2/Vs), the poly-Si TFTs can be applied to high quality displays for which existing a-Si TFTs are unsuitable. In addition, the degradation of poly-Si TFTs is much less than that of a-Si TFTs. However, in order to manufacture poly-Si TFTs, more processes are involved than those needed to manufacture a-Si TFTs and, additional equipment is also required. Accordingly, since poly-Si TFTs are economically inferior to a-Si TFTs, the poly-Si TFTs suffer from limitations as well. Since poly-Si TFTs have not been manufactured on a large substrate exceeding 1 m in width or size yet because of technical problems, for example, insufficient equipment or poor uniformity, the application of poly-Si TFTs to TVs is problematic, thereby making it more difficult for high performance poly-Si TFTs to survive and prosper in the market.
Accordingly, the demand for TFTs having a large design, low cost, and high uniformity, which are the advantages of a-Si TFTs, and high performance and high reliability, which are the advantages of a poly-Si TFTS, is the highest ever and research into the development of TFTs incorporating all of these elements is ongoing. One focus of research is on the development of an oxide semiconductor. Recently, ZnO-based TFTs have received increasing attention as oxide semiconductor devices. Different types of ZnO-based TFTs include a zinc oxide (ZnO) TFT, and a gallium-indium-zinc-oxide (“GIZO”) TFT that is a mixture of gallium oxide (Ga2O3), indium oxide (In2O3), and ZnO. According to Korean Patent Laid-Open No. 2004-0106576 and Korean Patent Laid-Open No. 2006-0123765, a ZnOx TFT, which is polycrystalline, has a high electron mobility similar to that of a poly-Si TFT but has low uniformity, while a GIZO TFT, which is amorphous, has higher characteristics than those of a-Si TFT. Accordingly, the GIZO TFT offers both the advantages of the a-Si TFT and the poly-Si TFT because the GIZO TFT is manufactured in the same manner as the a-Si TFT, thereby standing out as the optimal device. However, a well-defined method for manufacturing a GIZO TFT has not been developed, as there are several technical problems in the manufacturing processes. The production of bottom gate back channel etching (“BCE”) TFTs is preferred for several known reasons.
Since a GIZO semiconductor film is amorphous, the GIZO semiconductor film can be processed at a low temperature and can be easily made on a large scale. This aspect of the GIZO TFT is advantageous as the physical and electrical properties of the ZnO-based semiconductor film are greatly affected by thermal and chemical shock. When a ZnO-based TFT is manufactured, the ZnO-based semiconductor film is exposed to high-energy plasma.
Further, the carrier concentration in ZnO-based semiconductor films, including the GIZO semiconductor film, is sensitive to a change in oxygen concentration. Presently, the semiconductor film experiences an oxygen vacancy due to the decomposition of ZnO, resulting in an increase in the carrier concentration. The undesirable increase in the carrier concentration shifts the threshold voltage of the ZnO-based TFT to a more negative value, such that a large leakage current flows between the source electrode and the drain electrode even when a gate voltage is 0 Volts (V). The defect of a channel layer shifting the threshold voltage is related to the increase in the carrier concentration in the channel layer. Accordingly, there is a demand for a method of manufacturing a ZnO-based TFT having the desired electrical properties, by controlling the carrier concentration.